1. Field of the Invention
The present invention relates to a fluid dynamic bearing apparatus, a spindle motor, and a disk drive apparatus.
2. Description of the Related Art
Spindle motors arranged to rotate disks are typically installed in hard disk apparatuses and optical disk apparatuses. Such a spindle motor includes a stationary portion fixed to a housing of the apparatus, and a rotating portion arranged to rotate while supporting the disk(s). The spindle motor is arranged to produce a torque by magnetic flux generated between a stator and a magnet, whereby the rotating portion is caused to rotate with respect to the stationary portion.
The stationary portion and the rotating portion of the spindle motor are joined to each other through a fluid dynamic bearing. A known spindle motor including a fluid dynamic bearing is described, for example, in WO 2009/145159. A rotating member of the spindle motor described in WO 2009/145159 includes a communicating hole. In addition, a minute gap defined between a stationary portion and a rotating portion and the communicating hole of the rotating member are filled with a lubricating oil. Moreover, the lubricating oil has a pair of liquid surfaces defined at upper and lower positions, respectively.
In a spindle motor of this type, the lubricating oil may be intentionally caused to circulate, causing the lubricating oil to flow from a minute gap defined between a shaft and the rotating member into the communicating hole defined in the rotating member and then back into the minute gap, in order to discharge any air bubble introduced into the lubricating oil to an outside. However, depending on the shape of an end opening of the communicating hole, a high channel resistance may occur. A higher channel resistance increases a loss coefficient when the lubricating oil flows into the communicating hole, making it harder for the lubricating oil to flow into the communicating hole. If this happens, the amount of a portion of the lubricating oil which flows toward a liquid surface of the lubricating oil instead of flowing into the communicating hole is increased. This causes a rise of the level of the liquid surface of the lubricating oil, increasing the likelihood that a leakage of the lubricating oil to the outside will occur.